urlutil: add a new `get_unique_push_path` This function is dedicated to call that needs a single destination. Currently most caller actually need that since few actually support multiple destinations (the most importants `hg push` and `hg outgoing` do). So having a clear API point for that will be important when the time comes to have a single `[paths]` alias resolving to multiple urls. Differential Revision: https://phab.mercurial-scm.org/D10407